I could not reproduce the issue using your same code. I was able to successfully run your code. There where some weird embedded tabs but that was probably just a paste from this forum page. Make sure you have the field name correct. It defaults to the first field if the name you specify is not valid. If this used to work and no longer does it might be data specific. It would be best to open an issue with Esri support to get help developing a reproducible case that I can investigate.
Jeff - Layout and arcpy.mp teams